Method for selecting a restoration path in a mesh network

ABSTRACT

A method of selecting a restoration path in a mesh telecommunication network is disclosed that advantageously is practical and flexible and may be pre-computed along with a service connection path during the setup of the connection. The information used to select the restoration path can be advantageously distributed among nodes in the network.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior application Ser. No.09/909,102 filed Jul. 19, 2001 now U.S. Pat. No. 6,982,951, which claimspriority to United States Provisional Patent Application “A DISTRIBUTEDAPPROACH FOR RESTORABLE OPTICAL NETWORKS,” Ser. No. 60/257,029, filed onDec. 21, 2000, the contents of which are incorporated by referenceherein.

REFERENCED-APPLICATIONS

This application is related to U.S. Utility Patent Application, “METHODSAND SYSTEMS FOR FAST RESTORATION IN A MESH NETWORK OF OPTICAL CROSSCONNECTS,” Ser. No. 09/474,031, filed on Dec. 28, 1999, which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

The invention relates to telecommunications networks, and moreparticularly to selecting restoration paths in a telecommunicationsnetwork.

Modern telecommunication networks are reconfigurable and provide forfast restoration from network failures. For example, and in the contextof optical networking, Synchronous Optical Network (SONET) rings providethe primary technology for optical layer communication and restorationfrom network failures. SONET rings tend to be capacity inefficient whencompared to “mesh” topologies in networks with a high degree ofconnectivity and when, because of size limitations, connections areforced to route through many interconnected rings. As optical-crossconnects (OXCs) are deployed within today's transport networks based onwavelength-division multiplexing (WDM), the potential emerges to provideon-demand establishment of high-bandwidth connections (also referred toin the art as “lightpaths”). Emerging standards such as Multi-ProtocolLambda Switching (“MPL(ambda)S”) provide a standardized optical networkcontrol plane that is essential for building an effective platform forvendor interoperability. See, e.g., D. Awduche et al., “Multi-ProtocolLambda Switching: Combining MPLS Traffic Engineering Control withOptical Crossconnects,” IETF Internet Draft,http://www.ietf.org/internet-drafts/draft-awduche-mpls-te-optical-01.txt(November 1999). Unfortunately, few recent contributions to the art haveaddressed the need for fast failure restoration in such networks.

In co-pending commonly-assigned U.S. Utility Patent Application,“METHODS AND SYSTEMS FOR FAST RESTORATION IN A MESH NETWORK OF OPTICALCROSS CONNECTS.” Ser. No. 09/474,031, filed on Dec. 28, 1999, which isincorporated by reference herein, a restoration methodology is disclosedthat utilizes pre-computed restoration routes disjoint from the normalcommunication path—but wherein the channels/wavelengths may be chosendynamically during the restoration process. The invention thereindisclosed can potentially provide restoration competitive with SONETring restoration speeds. There is, however, still a need for a flexibleand practical methodology for selecting an advantageous restoration paththat may be utilized in a restoration process such as the one disclosedin the above application. Moreover, there is a need for a distributedapproach to restoration that permits the information needed forrestoration path selection to be distributed throughout the network witha minimum amount of signaling overhead.

SUMMARY OF THE INVENTION

A method of selecting a restoration path in a mesh telecommunicationnetwork is disclosed that advantageously is practical and flexible andmay be pre-computed along with a service connection path during thesetup of the connection. The restoration path is selected from a graphof links in the network which are physically diverse from the servicepath. For example, in the context of optical networking, the links donot share a common fiber span with the service path. Weights arecomputed for the links using an array representing a restoration linkcapacity—which is expressed as a number of channels/wavelengths inoptical networking—needed on each link over possible failures of theservice path. The restoration path is selected by minimizing the weightsfor each link in the restoration path. In accordance with another aspectof the invention, the information used to select the restoration path isadvantageously distributed among nodes in the network. A source nodeselecting a service path through the network to a destination node sendsa first message along the service path to the destination node, therebysetting up cross-connections for the service path. The destination nodesends a second message back to the source node along nodes in theservice path responsible for maintaining link information, whereby thenodes update the message with an array representing a restoration linkcapacity (channels/wavelengths) needed on each link over possiblefailures of the service path. The array is used by the source node toselect the restoration path. The source node then sends a third messagealong the service path and the restoration path to reserve resources.This provides a distributed approach for establishment of restorableconnections in a dynamically reconfigurable mesh network with a minimumamount of signaling overhead. Restoration paths are advantageouslydiverse from service connection paths while restoration link capacityadvantageously may be shared among different connections that wouldre-route over the restoration path during non-simultaneous failures.

These and other advantages of the invention will be apparent to those ofordinary skill in the art by reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an interconnection of optical cross-connects andoptical transport systems.

FIG. 2 and FIG. 3 are conceptual representations of the optical networkand the fiber span network, as shown in FIG. 1.

FIG. 4 is a flowchart of processing performed in computing a restorationpath through the network.

FIG. 5 is a flowchart of processing performed in distributinginformation for path selection in the network.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a mesh network 100, illustratively an optical network,organized into a general topology of links and nodes 110, . . . , 190,in contrast to a rigid ring or chain or other such structure. Althoughdepicted in FIG. 1 and described herein particularly in the context ofoptical networking, the invention can be readily appreciated by those ofordinary skill in the art to apply to cross-connect technologies ingeneral, e.g. Gigabit Ethernet, OXCs with electrical fabrics, etc.

With reference to FIG. 1, optical mesh network 100 comprises opticalcross-connects (OXCs) and optical transport systems (OTSs). The opticalcross-connects (depicted as boxes labeled “O” in FIG. 1) provideend-to-end optical connections, sometimes colloquially referred to as“lightpaths.” The optical transport systems in FIG. 1 comprise pairs ofbidirectional Wavelength Division Multiplexer (WDM) terminals (depictedas boxes labeled “W”) and amplifiers at intermediate locations whereneeded (depicted as boxes labeled “A”). The WDM terminals multiplexoptical signals at different wavelengths into a single optical fiber foreach direction of transmission using gratings or some equivalenttechnology. Each signal multiplexed by the optical transport system istraditionally referred to as an optical “channel”, each channeltransporting a signal (e.g., SONET OC-48/192, SDH STM-16/48, GigabitEthernet) usually associated with a link between nodes of higher-layertelecommunications networks such as routers or ATM switches. The signalsbetween the OXC and the WDM terminals are typically converted to ashorter, standard cross-office wavelength by optical transponders, notshown in FIG. 1, usually to avoid wavelength translation within the OXC.Collections of optical fibers that are co-located in the same cable,conduit, or substructure between two consecutive points of access—suchas a manhole, central office, or amplifier site—are referred to hereinas “fiber spans.” An “optical link”, as that term is utilized herein, isused to refer to a collection of channels that route over the same fiberspans between a pair of OXCs. Accordingly, mesh network 100 may berepresented as consisting of multiple layers: an “optical” network graphand a “fiber span” network graph illustrated by FIGS. 2 and 3respectively. The fiber span network is the graph G_(s)=(N_(s), E_(s)),where N_(s) is the set of nodes—that represent locations, generallywhere WDM equipment can be placed—and E_(s) is the set of fiber spans.Let N₀ be the set of the OXC nodes where N₀⊂N_(s). The graph of theoptical network is G₀=(N₀, E₀), where N₀ is the set of OXC nodes and E₀is the set of optical links which are routed on the fiber span networkof G_(s).

Each OXC has an associated optical layer control function, integratedwith the OXC or residing physically on a separate controller, whichcontrols the cross-connections of the OXCs and which can be used tocommunicate with the other OXCs. See, e.g., co-pending commonly-assignedU.S. utility patent applications, “METHOD AND APPARATUS FOR ROUTINGINFORMATION OVER OPTICAL AND ELECTRICAL PATHWAYS,” Ser. No. 09/685,953,filed Oct. 12, 2000; “CONTROL OF OPTICAL CONNECTIONS IN AN OPTICALNETWORK,” Ser. No. 09/769,735, filed Jan. 26, 2001; and “CONTROL OFOPTICAL CONNECTIONS IN AN OPTICAL NETWORK,” Ser. No. 09/769,780, filedJan. 26, 2001, which are incorporated by reference herein. It isassumed, without limitation, that the optical layer control function isdistributed. The cross-connection controllers can communicate with oneanother across a dedicated signaling channel (or a separate signalingnetwork) using known protocols such as Open Shortest Path Forwarding(OSPF). See, e.g. J. Moy, “OSPF Version 2,” IETF Network Working Group,RFC 2328 (April 1998); R. Coltun, “The OSPF Opaque LSA Option,” IETFNetwork Working Group, RFC 2370 (July 1998). OSPF link stateadvertisement (LSA) messages can be utilized to disseminate topology andchannel usage Information. It is advantageous to configure one OXC as a“master” OXC node for each fiber span and optical link, therebyallocating the control of the fiber span and optical link data among theOXCs in an unambiguous way. The master nodes are responsible formaintaining the link information. For each fiber span, if there is anOXC present at both terminating nodes of the fiber span, one canarbitrarily configure the OXC with lexicographically or numericallyhigher ID as the master. If there is only one OXC node present at aterminating node of the fiber span, then this node can be designated themaster. If neither node has an OXC present, it is necessary to configurea neighbor OXC node as the proxy master of the fiber span. See tables inFIG. 2 and FIG. 3 for examples. The two OXC nodes for each optical linkshould maintain information about the fiber spans which the optical linkpasses through and the masters of the fiber spans. All the informationcan be static and provided by initial configuration.

It is assumed that mesh network 100 provides for restorable connectionsthat may be established and torn down dynamically. Selecting optimalrestoration parameters involves a variety of considerations andobjectives.

First, it is advantageous to select a restoration path P_(r) that isphysically “diverse” from the service connection path P_(s). In otherwords, the restoration path and the service connection path should notbelong to a group of links (referred to in the art as a “shared risklink group”) sharing some common infrastructure that could subject thelinks to a possible single failure, e.g. a backhoe cutting a singlefiber conduit. Consider FIG. 2, the optical link map representation ofFIG. 1, which has six OXC nodes (the location nodes in FIG. 1 deployedwith OXCs) and ten optical links indexed as set forth in the table. Ifthe service path for a connection from OXC node A (210) to OXC node C(230) routes directly over the optical link from A-C (optical linkID=3), then it appears from FIG. 2 that the restoration path from OXCnode A (210) to OXC node B (220) to OXC node C (230) (link IDs 0-2-1)would be adequate. An examination of FIG. 3, the fiber span network,however, reveals otherwise. In FIG. 3, which has eight location nodesand nine fiber spans indexed as set forth in the table, it is clear thatthe restoration path A-B-C is not diverse from the service path A-C.Accordingly, in selecting a restoration path, it is important toconsider the routing of optical links over fiber spans—and to avoid, iftopologically possible, the sharing of any common fiber span.

Second, it is advantageous to share restoration channels on a commonlink of multiple restoration paths, in particular where fiber spanfailures are non-simultaneous. Unused channels may be reserved, i.e. notused for service paths, to ensure that adequate restoration capacity isavailable upon failure (alternatively, a dedicated restorationconnection—referred to in the art as “1+1” protection—can be utilizedalthough this tends to be impractical except for some high-priorityservices). A single restoration channel on a common link of multiplerestoration paths can be shared by non-simultaneous fiber span failures.For example, a channel can be used on a link of restoration path P_(r1)to reroute service path P_(s1) due to failure of fiber span i alongP_(s1), That same channel can be used to reroute service path P_(s2) torestoration path P_(r2) due to failure of fiber span j along P_(s2) aslong as failures i and j do not occur simultaneously. On the path ofP_(r), enough channels should be reserved such that in any single fiberspan failure, there are enough channels to restore all failed servicepaths. The total reserved channels, however, should be as small aspossible.

In accordance with an aspect of the invention, the restoration path isselected in a manner that permits restoration channels to be dynamicallyassigned and shared when a given fiber span fails. The restoration pathselection method can be utilized, without limitation, with a restorationarchitecture as disclosed in co-pending, commonly-assigned U.S. UtilityPatent Application, “METHODS AND SYSTEMS FOR FAST RESTORATION IN A MESHNETWORK OF OPTICAL CROSS CONNECTS,” Ser. No. 09/474,031, filed on Dec.28, 1999, which is incorporated by reference herein. As disclosedtherein, pre-computed restoration paths may be stored at the endpointnodes of the connection and utilized, upon a network failure, to reroutethe service connection. Although described with particular reference tothe restoration method described therein, one of ordinary skill in theart would readily recognize how to utilize the invention with otherrestoration methods. Requests to establish optical connections arrive atan OXC typically from a higher layer node (e.g., an Internet Protocol(IP) layer or element management system) via a User Network Interface(UNI). See, e.g. McAdams, L. and J. Yates, eds., “User to NetworkInterface (UNI) Service Definition and Lightpath Attributes,” OIFArchitecture Group, OIF2000.61 (2000). Each optical connection requestmay include restoration options, such as whether the connection shouldbe restorable if it fails due to a network failure and, if so, whetherthe channels of the restoration path are dedicated to this connection orshared among other connections that fail from other potential,non-simultaneous failures. If shared, the network resources can be usedmore efficiently because fewer total channels need to be reserved forrestoration. For example, a connection request, V, can be represented inthe form <source, destination, restoration-type, size> where source isthe origin OXC ID, destination is the terminus OXC ID, restoration-typeis the type of restoration capability required for the connection (e.g.1+1, non-restorable, mesh-restorable), and size is the bandwidth neededfor the connection (e.g. represented as the number of channels neededfor the connection).

The process of computation of service path and restoration path for aconnection request relies on the information about the availability ofoptical network resources and the path selection objective. A generalheuristic is to create some cost metric and select a “minimum weight”path among all suitable paths that minimizes the cost metric and has therequired size for the connection request. Additionally, severalinformation metrics are involved in the path selection process whenrestoration resources may be shared between different restoration paths.For example, each link has a maximum number of installed channels, perthe link augmentation (planning) processes: of that total, some channelsare assigned to service paths while other channels are reserved forrestoration paths. The remaining channels are unassigned and free to beallocated to new connections. Changes in the metrics need to beadvertised, e.g. as part of LSAs in extended OSPF, so that accurateinformation is available to every node.

SERVICE PATH. Selecting a service path in response to the communicationrequest, accordingly, may be accomplished by computing a path betweenthe source and destination that minimizes some cost metric and which hasthe required size for the connection request. It is assumed that eachOXC node has knowledge of the whole optical network topology and thenumber of free channels on each link as well as some optical link weightfunction. A known shortest path algorithm such as Dijkstra's shortestpath algorithm may be used to compute the minimal weight path throughthe network.

RESTORATION PATH: Selecting a restoration path in response to acommunication request requiring a mesh-restorable connection is morechallenging. It should be noted that the restoration path may not be the“min weight” path in any typical sense, because it can share restorationresources with other restoration paths if shared restoration isrequired. Note that although to provide clarity with the example, E₀ wasdefined above as the set of fiber spans, without loss of generality itcan also represent any set of shared risk link groups (“SRLGs”) overwhich one wishes to provide restoration. It is advantageous to definethe following parameters:

-   -   1. G₀, the optical network topology.    -   2. The relationship of links to fiber spans.    -   3. S_(k), the total number of channels reserved for use by        service paths for each optical link kεE₀.    -   4. F_(k), the number of channels “in service” (e.g. assigned to        working connections). The number of channels free for new        connection requests is therefore S_(k)-F_(k).    -   5. R_(k), the total number of channels reserved for restoration        on each optical link kεE₀.    -   6. failneed_(sk), the number of reserved, restoration channels        needed on optical link kεE₀ to reroute all failed connections        when fiber span sεE_(s) fails. These channels would be used by        the OXCs to reroute the connections (whose service paths route        over fiber span s) to their restoration paths.    -   7. maxfailneed_(k), the minimum number of reserved restoration        channels needed on optical link kεE₀ to meet a restoration        objective to restore 100% of connections that fail from any        single fiber span failure, e.g.        maxfailneed_(k)=max{failneed_(sk): fiber span sεE_(s)}. Note        that to meet the restoration objective R_(k)≧maxfailneed_(k),        i.e., at least this much restoration capacity must be installed.    -   8. M_(k), the number of channels needed on fiber link k over all        possible failures of the previously selected service path for V,        i.e., M_(k)=max {failneed_(sk): fiber span s is on the service        path for connection request V}.    -   9. Routing weights, w_(k), and initial weights, w_(0k), of        link k. These quantities are used as a measure of the heuristic        “cost” to the network. A path with a smaller total weight is        typically preferable. The initial weights w_(Ok) are initialized        to a value reflecting the hop count (e.g., w_(0k)=1) or        approximate cost of a channel (e.g., a pro-rated approximation        of the cost of OTS and OXC ports).        The parameters S_(k) and R_(k) can be determined by a capacity        planning and augmentation process. Note that the quantities        S_(k) and R_(k) have been broken out for clarity herein;        nevertheless, it is also possible to let the channels for        service paths and restoration paths be chosen from a combined        pool of free channels.

FIG. 4 is a flowchart of processing performed in computing a restorationpath through the network, in accordance with a preferred embodiment ofthis aspect of the invention. As described above, the restoration pathshould be physically diverse from the service path, e.g.fiber-span-disjoint. Such links may be identified manually or by using amethod such as the one disclosed in co-pending commonly-assigned utilitypatent application, “SYSTEM AND METHOD FOR AUTO DISCOVERY OF RISK GROUPSIN OPTICAL NETWORK,” Ser. No. 09/714,970, filed Nov. 20, 2000. At step401, all links which are not physically diverse from the service pathare deleted from the network topology graph, e.g. all links which shareat least one fiber span with the links in the service path. Then, atsteps 402 through 406, the weights of the remaining links are computed.If dedicated restoration is specified at step 403, then the weight isset to the available free channels on each link at step 404. If sharedrestoration is specified at step 403, then the restoration capacity canbe shared by multiple restoration paths if their service paths do notshare a common fiber span. When computing the restoration path, eachlink in the network must be weighted to reflect capacity sharingaspects. For shared restoration channels, the weights can be computed asshown at step 405, where C_(k)=maxfailneed_(k)-M_(k) and represents the“spare” restoration channels on optical link k. That is, if therestoration path for connection V is routed over optical link k, and ifthe connection size does not exceed C_(k) channels, then the totalrequired restoration channels (also including failures other than theservice path) on link k does not increase. The parameter ε is a smallpositive value which is defined to be much less than the weight on alink. The weight is set to ε rather than zero so that the algorithm willnot reuse capacity on shorter rather than longer paths. If there isinsufficient reserved restoration capacity on the link, the link must beavoided by setting its weight to infinity. Furthermore, the weight isset to infinity if the optical link k has failed or if the controlfunction for one of its end OXCs is failed. At step 407, the weights onthe remaining links are used to compute the restoration path using someshortest path algorithm. e.g. Dijkstra's shortest path first algorithm.

All of the information needed for the computation of the service andrestoration paths could be maintained at every OXC node. This wouldrequire maintaining the entire two-dimensional array, failneed_(sk), ateach OXC node. Whenever a new connection is provisioned, the matrixwould need to be updated, e.g. by flooding of link state advertisementmessages. In accordance with another aspect of the invention, however,portions of the array can be distributed around certain nodes wherebythe array portions are updated along the restoration and service pathsduring connection establishment. This serves to avoid flooding andminimizes the storage requirements for this information. Thus, inaccordance with a preferred embodiment of this aspect of the invention,each cross connect stores only the one-dimensional arrays failneed_(s•)for each fiber span s for which it is a master node and failneed_(•k)for each optical link kεE₀ for which it is a master node.

FIG. 5 is a flowchart of processing performed in computing anddistributing information for the path selection computation in thenetwork. In accordance with a preferred embodiment of the invention,four signaling messages are sent. The particular signaling protocol usedis not important to the invention; although to simplify the description,it is assumed below that the RSVP signaling protocol is being utilized.See, e.g., R. Braden, et al., ed., “Resource ReSerVation Protocol(RSVP)—Version 1 Functional Specification,” IETF Network Working Group,RFC 2205 (September 1997). At step 501, the source OXC node computes theservice path P_(s), for example using the method described above. Atstep 502, the source OXC node sends a forward message, e.g. a RSVP“PATH” message, toward the destination OXC node along the route of nodesin P_(s). The nodes, e.g. OXCs, set up cross-connections in parallel asthe message traverses. Once the destination node receives the message,it replies at step 503 with a backward message, e.g. a RSVP “RESV”message, sent back towards the source node. The backward message is sentfrom destination to source along nodes that are master nodes of fiberspans of the service path. The message contains the M_(k) array which isupdated at each node which is a master of a fiber-span along the servicepath. At each such node (say, a master node of fiber span s) along thereturn path, the node will update the M_(k) array as follows:

-   -   for each kεE₀,        M_(k)←max{M_(k),failneed_(sk)}.        Once the message reaches the source OXC node, the M_(k) array is        complete. The backward message serves to calculate the M_(k)        array and acknowledges the completion of the connection when        received at the source node. At step 504, the source node        computes the link weights and then computes the restoration path        P_(r), e.g. using the method described above. This restoration        path information is passed at step 505 in a message from source        to destination along the same nodes of the backward path, i.e.,        each node which is the master node of a fiber-span along the        service path, P_(s). This message contains the chosen        restoration path (to be stored in the destination node) and is        used to update the failneed_(s•) and maxfailneed_(k•) arrays.        Each such master node, n, upon receiving the message, updates        the array failneed_(s•) as follows:    -   for each fiber span, s, in the service path for which n is a        master node,        -   for each optical link, k, contained in the restoration path,            failneed_(sk)←failneed_(sk)+size.            All other values of failneed_(sk) remain unchanged. At step            506, a message from source to destination along the OXCs of            the restoration path, P_(r), is sent. This message contains            the service path information P_(s) and is used to update the            failneed_(•k) and maxfailneed arrays at master nodes of            optical links along P_(r). The message travels along the            restoration path; and each master node, n, of an optical            link, k, along the restoration path updates its            failneed_(•k) array as follows:    -   for each fiber span, s, contained in the service path        failneed_(sk)←failneed_(sk)+size.        All other values of failneed_(sk) remain unchanged.        maxfailneed_(k) is then updated accordingly. Finally, at step        507, the updated values of maxfailneed_(k) (only for each fiber        link, k, in the restoration path) as well as the updated values        of F_(k)(only for each fiber link, k, in the service path) are        flooded to every node. Extended OSPF link-state advertisements        can be used to update the changed elements of the F and        maxfailneed arrays to each OXC in the network.

As an example of the processing performed in FIG. 5, consider aconnection to be established between OXC node A (210) and OXC node F(260). First, a service path is selected, e.g. P_(s)=(A-C-F). A PATHmessage is sent along the path A-C-F. When node F (260) receives thePATH message, it sends out a backwards RESV message from node F (260) tonode A (210). However, link F to C uses three fiber spans 1,3,5 withfiber span master nodes C (230), E (250), F (260) respectively. So nodeF (260) sends the RESV message along the path F-E-C-A to compute theM_(K) information. After node A (210) receives the RESV message, itcomputes the restoration path, for example P_(r)={A-D-F}. Then node A(210) sends two PATH messages from node A (210) to node F (260). One isforwarded along A-C-E-F with P_(r) information to store the restorationpath at node F (260) and update the failneed_(s•) arrays along the way.The last message is forwarded along the path A-D-F and contains P_(s)information to update the failneed_(•k) and maxfailneed_(k) arrays. Ofcourse, each link master node needs to update the number of freechannels and number of reserved channels and let OSPF to flood to allother nodes.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention. For example, thedetailed description has been presented particularly in the context ofan optical networking architecture; however, the principles of thepresent invention could be extended to other cross-connect technologies.Such an extension could be readily implemented by one of ordinary skillin the art given the above disclosure.

1. A method of selecting a physical restoration data path in a meshtelecommunication network comprising the steps of: deleting links in agraph of the network which represent physical data paths which are notphysically diverse from a service path; computing by a master opticalcross-connect a weight for each remaining link in the graph using anarray representing a restoration link capacity needed on each remaininglink over possible failures of the service path, the weight for eachremaining link based on capacity sharing with other restoration paths;and selecting the physical restoration data path that minimizes theweights for each link in the physical restoration data path.
 2. Theinvention of claim 1 wherein the restoration link capacity on a link maybe shared by non-simultaneous failures of other service paths.
 3. Theinvention of claim 2 wherein nodes in the network are cross-connects. 4.The invention of claim 3 wherein the nodes in the network are opticalcross-connects.
 5. The invention of claim 4 wherein links are notphysically diverse from the service path if they belong to a shared risklink group.
 6. The invention of claim 4 wherein links are not physicallydiverse from the service path if they belong to a same fiber span. 7.The invention of claim 1 wherein link information used to compute thearray is centralized in the network.
 8. The invention of claim 1 whereinlink information used to compute the array is distributed among aplurality of nodes in the network.